In recent years, electrophotographic techniques have been widely utilized in the fields of copying machines and laser beam printers, because they have the advantage of high speed and high quality printing.
As electrophotographic photoreceptors used in these electrophotographic techniques, there have previously been widely known electrophotographic photoreceptors using inorganic photoconductive materials such as selenium, selenium-tellurium alloys, selenium-arsenic alloys and cadmium sulfide. On the other hand, compared to the electrophotographic photoreceptors using these inorganic photoconductive materials, electrophotographic photoreceptors using organic photoconductive materials have the advantages of being inexpensive and being excellent in productivity and easiness of waste disposal. The research of such electrophotographic photoreceptors have also become active. In particular, organic photoreceptors of a functionally separated laminate type comprising charge-generating layers for generating charge by exposure and charge-transporting layers for transporting charge, the charge-generating layers and the charge-transporting layers being laminated, are excellent in electrophotographic characteristics such as sensitivity, charging property and their stability on repeated use. For such organic photoreceptors, various proposals have hitherto been submitted and put to practical use.
When a photosensitive layer is formed by a coating procedure using a binder resin formerly proposed, an electrophotographic photoreceptor having somewhat sufficient durability can be obtained. The mechanical strength of its surface layer is however not necessarily sufficient. When the electrophotographic photoreceptor is repeatedly used in a copying machine for a long period of time, the surface of the photosensitive layer wears away, whereby the film thickness of the photoreceptor varies to decrease the sensitivity. As a result, there is the problem that fog is produced on copied images, or that charge potential is lowered to reduce the density of the copied images. For the electrophotographic photoreceptors, therefore, it has hitherto been desired to develop such a binder resin as forms a photosensitive layer having sufficient durability.
When such an electrophotographic photoreceptor is used in a high speed copying machine, not only the sensitivity of the electrophotographic photoreceptor is required to be high, but also the rapid decay of photoreceptor potential is needed. This means that it is necessary that carriers are rapidly injected from the charge-generating layer and the ability for transfer of the carriers in the charge-transporting layer is high. However, the charge-transporting layer formed by adding a charge-transporting material to the prior-art binder resin could not be said to have sufficient light decay characteristics compatible for all kinds of high-speed copying machines. It is the idea traditionally generally accepted that the transportability of the carriers is controlled by selection of the charge-transporting material and by the compounding amount thereof. Accordingly, the approach of finding out a binder resin effective to an improvement in charge transportability has not been sufficiently made.